In earth-boring operations, it is often necessary to straighten out the bore where the upper portion of the bore has sections with from moderate up to extreme curvature. In the past, if it were necessary to straighten out a well course below curved sections, the devices used had to be reamed through the curved sections. Often, the time required to ream was equivalent to the original drilling time for the section. This was expensive and sometimes resulted in sticking the drill string. Consequent expensive fishing jobs were required to free it. It is known that the predominant factors affecting stabilization or straightening of an earth bore are the proper articulation and geometric configuration of the stabilization assembly while maintaining an adequate or balanced force on the drill bit, since the curvature of the bore is determined primarily by the tilt of the bit within the bore.
It has been determined previously that in directional drilling operations the radius of curvature of the bore or drainhole to be drilled is related to the geometric configuration of the drilling assembly by the formula RC=L.sup.2 /2a where RC is the radius of curvature of the curved bore section, L is the distance between the pivotal axis of the lowermost articulated joint and tip of the bit or reamer and a is the distance between the center line of the curved bore section and the center line of the universal joint. For instance, essentially the same relationship was recognized and discussed in my prior U.S. Letters Pat. No. 3,398,804 and was employed in connection with directional drilling operations in the selection of the universal joint and reamer in establishing a predetermined radius of curvature. I have discovered that a similar principle is applicable to stabilization or straightening of curved bores where the distance "a" defines the difference between the hole radius and stabilizer radius and "L" defines the distance between the tip of the drill bit and the trailing end of the stabilizer in contact with the wall of the hole. From this formula, it can be readily appreciated that the radius of curvature RC can approach infinity and the curvature approximate a straight line if the distance "a" is reduced to a very small value or "L" increased, or a combination of both.
In the application of this principle to stabilization assemblies, it is important that the stabilizer members be so positioned and dimensioned in relation to the drill bit as not to cut into or ream the wall of the hole. This minimizes adverse steering and the amount of torque required in drilling. To do this, it is important to reduce the "change angle" forces on the stabilizer assembly to a minimum, particularly on that stabilizer nearest to the drill bit. It is also important to reduce the effects of deflection on the drill collar between the drill bit and stabilization assembly resulting from the curvature of the bore already drilled by making that member as "stiff" (i.e. large in diameter) as possible. Moreover, the distance L can be varied by shifting the contact area between the external surface of the stabilizer and the curved wall of the bore, since the effective distance L from the tip of the bit will extend to the trailing end of the first contact area of the stabilizer.
Representative patents of interest showing different types of stabilizer arrangements are U.S. Letters Pat. Nos. 496,316 to Mack; 1,971,480 to Earley; 2,669,429 and 2,669,430 to Zublin; 2,687,282 to Sanders; 3,156,310 to Frisby and 4,067,404 to Crase.